Genetics
- Monohybrid cross (Aa × Aa)
- genotypes 1 : 2 : 1, phenotypes 3 : 1
- Dihybrid cross
- phenotypes 9 : 3 : 3 : 1
- Hardy–Weinberg
- p + q = 1, p² + 2pq + q² = 1
- Independent genes
- multiply the probabilities
Biology: Genetics and Cell Biology: cram pack
Formula sheets, the key method steps for every topic and 4 practice problems, with full solutions at the back.
1Formulas and methods
2Practice problems
1.GeneticsMedium
Tall (A) is dominant over short (a), and round seeds (B) over wrinkled (b). The genes are on different chromosomes. For the cross AaBb × aabb, what is the probability P that an offspring is short, round seeds?
AaBb × aabb
P =
2.GeneticsHard
In a population of 10000 in Hardy–Weinberg equilibrium, 1 in 10000 show a recessive trait. How many individuals N are expected to be homozygous dominant?
N = 10000q² = 0.0001
N =
3.Cell biologyMedium
How do steroid hormones such as cortisol mainly act on their target cells?
- AThey cross the membrane and bind intracellular receptors that change gene transcription
- BThey activate adenylyl cyclase from outside the cell
- CThey are taken up by phagocytosis and digested
- DThey bind a membrane receptor and open ion channels, changing the cell's membrane potential
4.Cell biologyHard
Some antibiotics, such as tetracyclines, block bacterial protein synthesis with little effect on human cytoplasmic ribosomes. Why?
- ABacterial 70S ribosomes differ in structure from eukaryotic 80S ribosomes
- BBacteria use DNA instead of mRNA as the template for translation on ribosomes
- CHuman cells do not make proteins
- DHuman ribosomes are inside the nucleus
3Solutions
1.Genetics
Answer: P = 0.25
Mendelian genetics: each parent passes on one allele of each gene, chosen at random. A Punnett square lists every combination of gametes, each equally likely. A dominant allele (capital letter) shows whenever it is present; a recessive one only in the homozygous form. Genes on different chromosomes are inherited independently, so their probabilities multiply.
Each gene on its own
P(short) = 0.5
P(round seeds) = 0.5
Multiply
P = 0.5·0.5 = 0.25
Answer
P = 0.25
2.Genetics
Answer: N = 9801
Hardy–Weinberg: in a large, randomly mating population with no selection, the allele frequencies p (dominant) and q (recessive), p + q = 1, give genotype frequencies p², 2pq and q².
Allele frequencies
q = √0.0001 = 0.01
p = 0.99
Homozygous dominant
p² = 0.9801
N = 10000·0.9801 = 9801
Answer
N = 9801
3.Cell biology
Answer: A · They cross the membrane and bind intracellular receptors that change gene transcription
Why this is the answer
Steroids diffuse through the membrane and bind receptors in the cytoplasm or nucleus. The hormone–receptor complex acts as a transcription factor.
Answer
A · They cross the membrane and bind intracellular receptors that change gene transcription
4.Cell biology
Answer: A · Bacterial 70S ribosomes differ in structure from eukaryotic 80S ribosomes
Why this is the answer
Bacterial ribosomes (70S: 30S + 50S) differ from cytoplasmic eukaryotic ribosomes (80S: 40S + 60S), so drugs can target them selectively.
Answer
A · Bacterial 70S ribosomes differ in structure from eukaryotic 80S ribosomes
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